Lumbar puncture and reading the cerebrospinal fluid

Exam technique

A needle placed in the lower back below the end of the spinal cord to measure pressure and take spinal fluid for testing.

Updated Nov 13, 2026

See it in 3D: L3 vertebra · L4 vertebra · L5 vertebra · Supraspinous ligament · Interspinous ligaments · Ligamentum flavum · Dura mater · Conus medullaris · Cauda equina

What it is

Lumbar puncture reaches the subarachnoid space below the end of the spinal cord, where only the nerve roots of the cauda equina float in the fluid. It measures the opening pressure and gives cerebrospinal fluid for cells, glucose, protein, stains and cultures. Its main uses are suspected meningitis or encephalitis, subarachnoid hemorrhage with a normal CT, Guillain–Barré syndrome, multiple sclerosis and idiopathic intracranial hypertension.

Landmark anatomy

In adults the spinal cord ends at about L1–L2 (the conus medullaris); below it lie the cauda equina and the dural sac, which reaches about S2.
The line joining the tops of the iliac crests (Tuffier line) crosses the L4 spinous process or the L4–L5 space.
The needle passes skin, subcutaneous fat, the supraspinous and interspinous ligaments and the ligamentum flavum, then the epidural space, the dura and the arachnoid.
A 'give' is often felt as the needle crosses the ligamentum flavum and again at the dura.

Technique, step by step

Check for contraindications first and obtain consent; explain the procedure to the patient.
Image the head before the puncture if there is papilledema, a focal deficit, reduced consciousness, a new seizure or immunosuppression.
Position: lying on the left side, curled with knees to chest and neck flexed; or sitting, leaning forward over a table.
Find the top of the iliac crests and the L4 spinous process; choose the L3–L4 or L4–L5 space in the midline.
Full sterile technique: skin antisepsis, sterile drape and gloves, then local anesthesia of the skin and track.
Advance the needle with its stylet in the midline, angled slightly toward the umbilicus, with the bevel facing the patient's side.
Remove the stylet to check for fluid; with the patient lying on the side and relaxed, measure the opening pressure with a manometer.
Collect the fluid in numbered tubes for cell count, biochemistry, microbiology and other tests; note its color.
Replace the stylet before withdrawing the needle; observe the patient and record headache, back pain or new neurological signs.

Cerebrospinal fluid patterns (typical; values overlap and vary by laboratory)

Pattern	Appearance and pressure	White cells	Glucose	Protein
Normal	Clear like water; normal pressure	Very few (under about five), lymphocytes	About two-thirds of blood glucose	Normal
Bacterial meningitis	Cloudy; high pressure	Hundreds to thousands, mostly neutrophils	Low (often under 40 percent of blood glucose)	High
Viral meningitis	Clear; normal or slightly high pressure	Tens to hundreds, mostly lymphocytes (neutrophils very early)	Normal	Normal or mildly high
Tuberculous or fungal (cryptococcal)	Clear or slightly cloudy; high pressure, often very high in cryptococcal disease	Tens to hundreds, mostly lymphocytes	Low	High, often very high in tuberculosis
Subarachnoid hemorrhage	Bloody fluid that stays bloody across tubes; yellow (xanthochromia) after several hours; high pressure	Many red cells; white cells in proportion to the blood	Normal	Mildly high

Abnormal findings

Albuminocytologic dissociation (high protein with a normal cell count) supports Guillain–Barré syndrome.
Oligoclonal bands confined to the fluid support multiple sclerosis.
High opening pressure with normal fluid and normal imaging supports idiopathic intracranial hypertension.
A traumatic tap gives blood that clears from the first to the last tube, without xanthochromia.
India ink, cryptococcal antigen and mycobacterial tests are requested when cryptococcal or tuberculous meningitis is suspected, both important in people with HIV in Latin America.

Common errors

Contraindications: suspected raised pressure from a mass or obstruction, skin infection at the site, bleeding disorder or anticoagulation, suspected spinal cord compression.
Measuring opening pressure with the patient sitting gives falsely high values.
Glucose in the fluid is only interpreted against a blood glucose taken at the same time.
Post-puncture headache, worse when upright and relieved lying down, is the most common complication.

Test yourself

Think of the answer, then tap the question to see it.

Why is lumbar puncture done at L3–L4 or L4–L5 in adults?

Because the spinal cord ends at about L1–L2; below it the needle meets only the floating roots of the cauda equina.

When is brain imaging needed before a lumbar puncture?

When there is papilledema, a focal deficit, reduced consciousness, a new seizure or immunosuppression, to rule out a mass that could cause herniation.

What CSF pattern suggests bacterial meningitis?

Cloudy fluid with high pressure, many neutrophils, low glucose and high protein.

How is a traumatic tap told apart from subarachnoid hemorrhage?

In a traumatic tap the blood clears from the first to the last tube and there is no xanthochromia; in hemorrhage it stays bloody and turns yellow after several hours.

What is albuminocytologic dissociation?

High protein in the spinal fluid with a normal white cell count, typical of Guillain–Barré syndrome.

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Connections

← All of the clinical exam

Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.

Sources: Lumbar Puncture (MSD Manual Professional Edition, 2024); Cerebrospinal fluid (CSF) collection (MedlinePlus, U.S. National Library of Medicine, 2024).

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